The present invention relates to a source driver, an electro-optical device and an electronic apparatus having the source driver, and a driving method.
Simple matrix liquid crystal display panels and active matrix liquid crystal display panels using switching elements, such as thin-film transistors (TFT), are known as liquid crystal panels (electro-optical devices) used in cellular phones and other electronic apparatuses.
The simple matrix method can reduce power consumption easily compared to the active matrix method, but is not suited to display multiple colors and moving images. Meanwhile, although the active matrix method is suited to display multiple colors and moving images, the method is not good for reducing power consumption.
The demand for multi-color, moving imaging has increased in recent years to provide high quality images with cellular phones and other mobile electronic apparatuses. In response to this demand, active matrix liquid crystal display panels are increasingly replacing simple matrix liquid crystal display panels.
To drive an active matrix liquid crystal display panel, an impedance conversion circuit functioning as an output buffer is provided in a source driver that drives source lines of the display. As this impedance conversion circuit, an operational amplifier that has a connection to be a voltage follower circuit is adopted. This configuration provides high driving capability, but increases power consumption because of the operating current of the operational amplifier. In driving this kind of liquid crystal display panel, a method for turning part of its displayable area to a display status and other parts to a non-display status has been employed to reduce power consumption. Japanese Unexamined Patent Application Laid-Open No. 11-184434 is an example of related art.
In an active matrix liquid crystal display panel including a plurality of source lines and a plurality of gate lines, display and non-display areas are set in a displayable area of the panel to provide a partial display. The display area is part of the displayable area that is turned to a display status, while the non-display area is other part that is turned to a non-display status. The two areas are divided by the source and gate lines. The display or non-display status of each area is set by a source driver for driving the source lines and a gate driver for driving the gate lines
In order for the source driver to provide a partial display divided by the source lines, the driver loads “off” display data for turning a non-display area to the non-display status, as well as display data for displaying an image in a display area. The source driver then drives some source lines in the display area based on the display data, and also drives other source lines in the non-display area based on the “off” display data. Accordingly, a voltage of the source lines is applied to a pixel electrode coupled to selected gate lines, and thus the display and non-display statuses can be set.
To provide a partial display divided by the gate lines, however, the gate driver outputs a selection voltage to some gate lines in a display area, outputs this selection voltage once to other gate lines in a non-display area, and then needs to control not to output this selection voltage again to the gate lines in the non-display area in and after the next frame. Therefore, the source driver has to drive source lines on one scan line every time irrespective of whether they are in the display or non-display area divided by gate lines. Consequently, the source driver consumes unnecessary power as it drives source lines in a non-display area although it is divided by gate lines.
An operational amplifier of an impedance conversion circuit for driving source lines is provided with a capacitor for preventing oscillation in its path in which its output is returned.
However, such a capacitor for preventing oscillation provided to the operational amplifier makes it difficult to reduce circuit size. When applying it as an output buffer to the source driver, in particular, one operational amplifier is provided for every 720 source lines, for example, which increases a chip area and cost.
In addition, the operational amplifier includes a differential amplifier and an output circuit, for example. The reaction (response) rate of the output circuit may be much higher than that of the differential amplifier. In this case, an increased load capacity decreases the reaction rate of the output circuit. As a result, the reaction rate of the output circuit becomes closer to that of the differential amplifier, and thereby oscillation becomes likely. This means that oscillation margin becomes low, since a larger liquid crystal panel increases the output load of the operational amplifier.
Furthermore, it is necessary to change a capacity of the capacitor for preventing oscillation. Therefore, providing such a capacitor inside a circuit makes it necessary to provide an extra switching element for trimming the capacitor, and deteriorates characteristics of the capacitor itself.
In consideration of the need for less costly, larger liquid crystal panels, the voltage follower circuit preferably has a lower phase margin with load unconnected to its output than with load connected to its output. Accordingly, there is no need to provide the capacitor for preventing oscillation. As a result, a phase margin increases as the size of a liquid crystal panel increases and its output load increases, and thereby oscillation can be prevented.